Radiation detection for soft x-rays

ABSTRACT

The apparatus comprises a proportional counter with a long tubular nose-piece having an end window. The anode is a short wire parallel to the window and mounted on a support so that the support together with the anode is easily removable and replaceable. The support may be axially movable within the long tubular nose-piece, whereby sensitive tuning to specific characteristic X-rays is possible.

United States Patent Cairns et al.

RADIATION DETECTION FOR SOFI X- RAYS Inventors: James Anthony Caims,Didcot; Colin Leonard Desborough, Faringdon, both of England UnitedKingdom Atomic -Energy Authority, London, England Filed: June 1, 1970Appl. No.2 42,166

Assignee:

Foreign Application Priority Data May 30, 1969 Great Britain...'...27,660/69 US. Cl. ..'.......2s0/s3.6 R, 313/146 Int. Cl ..G01t 1/18Field of Search.....250/83.6 R, 43.5 D; 313/146,

its] 3,704,371 51 Nov. 28, 1972 [56] References Cited 7 UNITED STATESPATENTS 3,449,573 6/ 1969 Lansiart et al. ...250/83.6 R X 3,110,83511/1963 Richter et al ..250/83.6 R X 3,091,716 5/1963 Engelmann..3l3/l46 Primary Examiner-James W. Lawrence Assistant Examiner-Davis L.Willis Attorney-Larson, Taylor and Hinds [57] ABSTRACT The apparatuscomprises a proportional counter with a long tubular nose-piece havingan end window. The anode is a short wire parallel to the window andmounted on a support so that the support together with the anode iseasily removable and replaceable. The support may be axially movablewithin the long tubular nose-piece, whereby sensitive tuning to specificcharacteristic X-rays is possible.

14 Claims, 4 Drawing Figures PATENTED NOV 28 m2 3.704.371 sumamzRADIATION DETECTION FOR SOFT X-RAYS BACKGROUND OF INVENTION Theinvention relates to apparatus. for detecting radiation, more especiallyX-radiation.

SUMMARY OF THE INVENTION The invention provides apparatus for detectingradiation, more especially X-radiation, which apparatus comprises anenclosure having a window,'that is a region (hereinafter referred to asa window) adapted for transmitting the radiation to be detected, a pairof electrodes one of which comprises an elongated member which extendsperpendicularly to the central normal to the window surface, and asupport for the said one electrode, the said support together with thesaid one electrode being removable as a unit from the enclosure.

Preferably the window is plane so that the elongated electrode extendsparallel with the plane of the window.

Preferably the other of the said electrodes has a configuration which issymmetrical about the said elongated electrode. Preferably the elongatedelectrodeis employed in operation as an anode and the said otherelectrode is employed as the cathode. Preferably the cathode encompassesthe anode, the said window being electrically conducting and formingpart of the cathode.

Preferably the enclosure is elongated, preferably tubular, and thewindow is located at an end of the elongated enclosure. Preferablyremoval and replacement of the combined support and anode unit iseffected by extracting the unit from the end of the elongated enclosureremote from the window. In this way, if the vacuum seal into a targetchamber is made onto the body of the said elongated enclosure, the anodemay be replaced without loss of vacuum.

It is an important feature of the arrangement according to the inventionthat an electrically conducting element located on the side of the anoderemote from the window is in electrical contact with, and forms partof,- the cathode.

Preferably the said electrically conducting element is comprised by thesaid support for the anode, there being insulating means between theelement and the anode.

Preferably means is provided for adjusting the separation of the saidone electrode from the window. Preferably the support and the said oneelectrode are slidable as a unit in the enclosure to effect theadjustment of the said separation.

BRIEF DESCRIPTION OF THE DRAWINGS Specific constructions of apparatusembodying the invention will now be described by way of example and withreference to the accompanying drawings in which:

The apparatus of these examples is similar in the general principles ofconstruction to the type of radiation detector described as aproportional counter. The apparatus will hereinafter be referred to as acounter.

The counter of the example shown in FIG. 1 is primarily intended for themeasurement of soft X-rays, that is X-rays of energy less than, say, 1keV.

Referring to the drawing, the counter comprises a long stainless steelouter tube 11 with an aluminized Mylar window 12 at one end. The window12 is secured by a threaded ring 13 which traps an O-ring seal 14 to Imake a vacuum-tight joint.

An anode wire 15 is supported on stainless steel posts 16 which meet ina Y junction (not visible in the drawing) and are themselves supportedby a hollow pin vice 17, one end of which clamps the stem of the Yjunction and the other'end 18 of which is secured to an inner tube 19 ofstainless steel.

The inner tube 19 extends through the counter to just beyond a locknut,reference 36, where it is connected to polythene tubing. The tubingemerges at 21 where it is connected, when the counter is in operation,to a supply of the counting gas.

The stainless steel posts 16 extend through a spider 22. The spider 22is metallic and adapted to engage with its fingering against theinternal wall of the tube 11. The spider 22 is insulated from the posts16 by insulating ceramic collars 23. The spider 22, the walls of theouter tube 11 and the window 12 are held at earth potential duringoperation and enclose the operating zone of the counter within which theanode 15 is situated. Holes 24 in the spider 22 admit gas fromthe innertube 19 into this zone. The gas passes back along the outer tube 1 1 toan outlet 25.

It will be appreciated that the spider 22 serves to locate the anode 15,to center the inner tube 19 and to provide an earthed screen behind theanode 15. Electrical connection to the anode 15 is made via the innertube 19.

At the end of the counter tube 11 remote from the window 12, the innertube 19 extends through and is located by a tubular perspex member 26.

The inner tube 19 and the, perspex member 26 together extend through arectangular housing 27 with a short hollow cylindrical nose 28 by whichit is sealingly joined by union nut 29 to the outer tube 1 1.

The perspex member 26 is fixed to a collar 31 which is grooved to permitpassage of gas around the outside of the perspex member 26 to the outlet25. The collar 31 moves with the perspex member 26 and has a keyengaging a groove within the nose 28 to prevent rotation. A bearing 32secured by union nut 33 where the perspex member 26 enters therectangular housing 27 has sealing rings 34, 35 to prevent the gas fromentering the rectangular housing 27, or to provide a vacuum seal when itis desired to operate the counter with the gas at low pressure bypumping out from 25.

The end of the perspex member 26 remote from the outer tube 1 1 isinternally and externally threaded. The internal thread co-operates withan external thread on the inner tube 19 and the perspex member 26 andinner tube 19 are locked together by lock nut 36. The external thread onthe perspex member 26 is engaged by an elongated spider 37 fixed to therotatable portion of a knob'38 mounted on the rectangular housing 27.The knob 38 is calibrated and, when rotated, moves the perspex member 26together with an inner tube 19 and anode l5 axially.

A head amplifier of thin film/LID construction is mounted on a circuitboard 39 which is accommodated within the rectangular housing]. Thecircuit board 39 is mounted by attachment to a clamp 41 on the perspexmember 26. Leads for connections to the amplifier, the anode (via a loadresistor 44 on the circuit board and inner tube 19) and the cathode(provided by the earthed body of the outer tube 11) are taken toplug/socket connectors 42.

It will be appreciated that if the knob 38 is appropriately rotated theperspex member 26, carrying with it the inner tube 19 and the anode 15,may be moved axially to adjust the spacing between the anode and thewindow 12. I

In FIG. 1, the counter is shown with the anode 15 .at the settingclosest to the window 12. This limit setting is provided by a collar 43clamped to the perspex member 26. At the other limit of axial movementof the perspex member 26, the steel inner tube 19 stops short of the endwall of the housing 27 to avoid the introduction of capacitative effectsbetween the inner tube 19 and the body of the knob 38. p

The circuit board 39 moves with the perspex member. This is importantfor minimizing the length of lead, and hence minimizing the capacity ofthe lead, in the connection between the anode l5 and the head amplifier.

In this example the window 12 is plane and an important feature of thearrangement of the counter is that the anode 15 comprises an extended orelongated member, the length of which is parallel to the window 12. Thisgeometrical arrangement is desirable for securing good resolution aswill be apparent from the following reasoning. Consider a band of X-raysof uniform energy entering the counter through the window 12. The X-rayswill all have the same mean path length in the gas of the counter andelectrons released by ionization of the gas at the ends of the X-raypaths will travel, under the electrostatic field, to the anode. For goodresolution, it is important that all the electrons resulting from X-raysof equal energy should reach the anode at the same time. It will beappreciated that a certain amount of curvature of the window surface maybe tolerated. In fact a curvature about the axis defined by the anodemight even be desirable but is not very practicable with the dimensionsof the counter of this example. However, taking possible curvature intoaccount, the relative orientation of window and anode may be defined inthat the anode extends perpendicularly to the central normal to thewindow surface.

In operation, the long, comparatively narrow tube 1 1 provides aconvenient probe by which the sensitive end, that is the window end, maybe positioned close to a target within a vacuum system. The vacuumsystem is sealed onto the outer tube 11 at any convenient point betweenthe union nut 29 and the window end of the outer tube 1 1.

The anode voltage is applied to the inner tube 19 and the surroundingcathode is earthed. Gas is passed through the tube 19 and is guided tothe sensitive zone so as to give most effective flushing out of thiszone.

X-rays passing through the window 12 ionize the gas and produce currentpulses between anode and cathode, these current pulses being detectedand amplified by the head amplifier. The amplitude of the pulsesisproportional to the energy of the X-rays and the count rate isproportional to the X-ray intensity.

The pressure of the counting gas within the outer tube .1 1 is set sothat the gas path length of the X-rays of the energy it is desired todetect is of the right approximate magnitude. The adjustability of theaxial position of the anode 15 of the apparatus then'provides for afiner setting to achieve optimum spacing of the anode from the window inrelation to the gas path length.

The apparatus of this example does, however, provide for an unexpectedlysensitive tuning of the counter to specific characteristic X-rays. Thus,for example, in a plot of count rate against pulse amplitude, the peaksfor the "characteristic X-radiation of silicon-K and copper-L, are veryclose together and a proportional counter has high resolution if theycan be clearly distinguished. It would not be expected that, for suchsimilar peaks, the setting of the anode position relative to the windowwould produce any differentiation in sensitivity to the X-rays of thesetwo elements. However, it has been found with the apparatus of thisexample that the counter can be tuned either to the copper-L peak or tothe silicon-K peak by adjustment of the knob 38. When set for maximumresponse to the X-rays from copper- L, the response to silicon-K X-rayswas of the order of five-fold less sensitive and vice versa. i

This phenomenon is believed to be due to the. geometry of thearrangement and the relative distribution of the electrostatic fieldaccording to whether the spacing between anode (which'extends parallelto the window) and the spider 22 is greater or less than the spacingbetween the anode and the window. The influence, upon the electricfield, of the metal washer 12a on the anode side of the window also hasa significant effect upon the distribution of the electrostatic field.It

is important that a cathode screen is positioned behind the anode 15,which screen is provided by spider 22 and moves with the anode 15 so asto have constant spacing therefrom. While the screen has holes 24 forpermitting the passage of gas, the number and size of these holes isminimized so that the screen is as complete and uniform as possible.

The above-described phenomenon of exceptionally sensitive selectivetuning of the counter may be undesirable or unnecessary in certainapplications. Accordingly FIGS. 2 to 4 illustrate a simplifiedconstruction in which no provision is made for adjusting the anodeposition and the window is adapted for mounting in a manner which avoidsthe sensitive selective tuning mentioned above. The remaining featuresof the design of FIG. 1 are substantially embodied and improved upon inthe apparatus of FIGS. 2 to 4 in a manner which provides an extremelycompact counter capable of insertion as a complete unit within a vacuumsystem.

Referring to FIGS. 2 to 4, a stainless steel outer tube 51 correspondsto the outer tube 11 of FIG. 1, but is ing off the inside walls of thethreaded ring and associated washer 53 and sealing ring trappingelements 54. It will be appreciated that such scattered X-rays would bedegraded in energy and would spoil the resolution of the counter. v

Further, it has been found that the washer 12a in the FIG. 1 arrangementdisturbs the uniformity of the electrostatic field. By securing anindium O ring with adhesive to the Melinex window 55, it has beenpossible to arrange the components as shown in FIG. 2 and the upper partof FIG. 4 with the window innermost.

The anode unit 56 corresponds closely in construction to the anode unitof FIG. 1 and is located so that the anode wire 57 is equidistant fromthe window 55 and the cathode screen provided by thesupporting spiderillustrated diagrammatically at 58.

This arrangement achieves a particularly uniform electric field andadjustment of the counter for detection of a range of X-ray energies isachieved in the conventional manner by adjusting the operating gaspressure. A degree of additional flexibility is provided by the facilityfor rearranging the window assembly in the manner illustrated in thelower part of FIG. 4.

The anode unit 56 is secured to a perspex block 59 which is mountedwithin a cylindrical stainless steel casing 61. Also housed within thecasing 61 is a circuit board 62 supporting the head amplifier. Seals areprovided at 63, 64, 65 and 66 so that the counter gas chamber and theenclosure of the head amplifier are separate and sealed. Thus, ifdesired, the whole unit can be placed in a vacuum system. Alternatively,the outer tube 51 only may be received in a seal into the vacuum system.In that case, as with the FIG. 1 arrangement, the anode unit can bereplaced without breaking the vacuum.

The gas path comprises inlet connector 67,'bore 68 in the casing, pipe69, polyvinylchloride connector 71, stainless steel inner tube 72 andthen via the anode unit into the counter chamber region within outertube 51. The return gas path is via grooves 73 in the perspex block,annulus 74 and bore 75 to outlet connector 76 (FIG. 3).

Electrical connector sockets are provided at 77, 78, 79 and connectionto the anode is via terminal 81 and the inner tube 72.

The overall length of this example is 4.5 inches (1 1.4 cms) and thediameter of the cylindrical end casing 61 is 3.25 inches (8.3 cms).

It will be appreciated that the modifications in shape and manner ofassembly of the window securing components, discussed in relation to theexample of FIGS. 2 to 4, may be applied to the example of FIG. 1.

The principal advantageous features of the apparatus of these examplesis that the complete anode unit may readily be removed and replaced inthe event of damage, without breaking the vacuum in the target chamber(except, of course, when using the apparatus of FIGS. 2 to 4 whollywithin the vacuum system). The window 12 can be very thin if desired,e.g. 0.0001 inch aluminized polycarbonate or 0.0002 inch aluminizedMylar, and is adequately supported in vacuum-tight seal without the needfor a grid support. Windows can readily be interchanged if desired.

The head amplifier unit and its associated electronic circuitry arebuilt inside the body of the counter to minimize capacitance of the leadto the anode, but can be replaced easily, again generally withoutbreaking the vacuum in the target chamber.

The invention is not restricted to the details of the foregoingexamples.

We claim:

1. A radiation detector comprising a first electrode of hollow form, awindow for transmitting radiation disposed at one end of the firstelectrode, a second electrode disposed within the first electrode, saidsecond electrode being of elongated form and extending lengthwise withinthe first electrode in a direction perpendicular to the central normalto the window surface and adjacent said window, electrically conductingsupport means for said second electrode slidably supported by the firstelectrode and in electrical contact therewith, and means insulating saidsupport means from said second electrode, the said support means andsaid second electrode being removable as a unit from said firstelectrode.

2. A detector as claimed in claim 1, wherein the window is plane so thatsaid elongated second electrode extends parallel with the plane of thewindow.

' 3. A detector as claimed in claim 1, wherein, in use, the saidelongated second electrode comprises an anode and the said hollow firstelectrode comprises the cathode, the said window. being electricallyconducting and forming part of the cathode.

4. A detector as claimed in claim 3, wherein the said hollow firstelectrode has a symmetrical configuratio about the anode.

5. Apparatus as claimed in claim 4, wherein the said hollow firstelectrode is elongated.

6. A detector as claimed in claim 1, provided with a cover demountablyconnected to the other end of the said first electrode so as to closesaid other end, and means connecting the unitary second electrode andsupport means to said cover so that said cover becomes part of the unit.

7. A detector as claimed in claim 3, wherein the said second electrodesupport means comprises a cathode shield.

8. A detector as claimed in claim 1, wherein means is provided foradjusting the separation of the said second electrode from the saidwindow.

9. A detector as claimed in claim 8, wherein the second electrodesupport means and the said second electrode are slidable as a unit inthe hollow first electrode to effect the adjustment of the saidseparation.

10. A detector as claimed in claim 8, wherein, in use, the saidelongated second electrode comprises an anode and the said hollow firstelectrode comprises the cathode, the said window being electricallyconducting and forming part of the cathode.

11. A detector as claimed in claim 10, wherein the said hollow firstelectrode is elongated.

12. A detector as claimed in claim 10, wherein the said second electrodesupport means comprises a cathode shield.

13. Apparatus for detecting radiation comprising an enclosure, a windowin the enclosure for transmitting radiation, a pair of electrodes one ofwhich comprises an elongated member which extends perpendicularly to thecentral normal to the window surface, and a support for the said oneelectrode, the said support sulating means between the element and theanode, the

element being located, in use, on the side of the anode remote from thewindow and forming part of the cathode.

14. Apparatus as claimed in claim 13, wherein means is provided foradjusting the separation of the said one electrode from the window.

1. A radiation detector comprising a first electrode of hollow form, awindow for transmitting radiation disposed at one end of the firstelectrode, a second electrode disposed within the first electrode, saidsecond electrode being of elongated form and extending lengthwise withinthe first electrode in a direction perpendicular to the central normalto the window surface and adjacent said window, electrically conductingsupport means for said second electrode slidably supported by the firstelectrode and in electrical contact therewith, and means insulating saidsupport means from said second electrode, the said support means andsaid second electrode being removable as a unit from said firstelectrode.
 2. A detector as claimed in claim 1, wherein the window isplane so that said elongated second electrode extends parallel with theplane of the window.
 3. A detector as claimed in claim 1, wherein, inuse, the said elongated second electrode comprises an anode and the saidhollow first electrode comprises the cathode, the said window beingelectrically conducting and forming part of the cathode.
 4. A detectoras claimed in claim 3, wherein the said hollow first electrode has asymmetrical configuration about the anode.
 5. Apparatus as claimed inclaim 4, wherein the said hollow first electrode is elongated.
 6. Adetector as claimed in claim 1, provided with a cover demountablyconnected to the other end of the said first electrode so as to closesaid other end, and means connecting the unitary second electrode andsupport means to said cover so that said cover becomes part of the unit.7. A detector as claimed in claim 3, wherein the said second electrodesupport means comprises a cathode shield.
 8. A detector as claimed inclaim 1, wherein means is provided for adjusting the separation of thesaid second electrode from the said window.
 9. A detector as claimed inclaim 8, wherein the second electrode support means and the said secondelectrode are slidable as a unit in the hollow first electrode to effectthe adjustment of the said separation.
 10. A detector as claimed inclaim 8, wherein, in use, the said elongated second electrode comprisesan anode and the said hollow first electrode comprises the cathode, thesaid window being electricalLy conducting and forming part of thecathode.
 11. A detector as claimed in claim 10, wherein the said hollowfirst electrode is elongated.
 12. A detector as claimed in claim 10,wherein the said second electrode support means comprises a cathodeshield.
 13. Apparatus for detecting radiation comprising an enclosure, awindow in the enclosure for transmitting radiation, a pair of electrodesone of which comprises an elongated member which extends perpendicularlyto the central normal to the window surface, and a support for the saidone electrode, the said support together with the said one electrodebeing removable as a unit from the enclosure, the elongated electrodecomprising an anode and the said other electrode comprising the cathode,the cathode encompassing the anode, the said window being electricallyconducting and forming part of the cathode, said support comprising anelectrically conducting element, there being insulating means betweenthe element and the anode, the element being located, in use, on theside of the anode remote from the window and forming part of thecathode.
 14. Apparatus as claimed in claim 13, wherein means is providedfor adjusting the separation of the said one electrode from the window.